Electronic and electro-hydraulic steering control systems are currently implemented in vehicle steering systems to take advantage of their versatility over mechanical and hydraulic steering systems. Such control systems commonly use hydraulic power as the muscle for the actual steering function and electrical components as the command for the precision control of the hydraulic components. These control systems are particularly advantageous in excavating machines where the hydraulic power is necessary to move the steering components of the machine. In the event of performance issues or failure of the primary electronic or electro-hydraulic steering control system, a redundant control system is typically implemented in concert with the primary system to take over control of the steering components when necessary.
One such steering control system is disclosed in U.S. Pat. No. 4,736,811 issued to Marsden et al. on Apr. 12, 1988. The steering control system for lightweight vehicles is electronically controlled in its primary operation, and has a manually operated mechanical control as a backup to the electronic controls. The steering control system includes an electrically controlled circuit actuated by rotation of a steering wheel for controlling a main pilot operated steering valve as the primary steering control. A hydraulic control circuit is also actuated by rotation of the steering wheel for controlling the same steering valve as a backup control when the electrically controlled circuit is disabled or otherwise not active. The hydraulic steering circuit is disabled when the electrically controlled circuit is activated. The same steering valve is an integral part of both control circuits.
Another example of a steering control system is disclosed in U.S. Pat. No. 6,712,176 issued to Zenker et al. on Mar. 30, 2004. The reference patent discloses a hydraulic dual circuit steering system with a first circuit having a first control unit and a first steering motor connected with the control unit via working connections, a second circuit having a second control unit and a second steering motor connected with the control unit via working connections, and a changeover valve. The changeover valve activates the first circuit and deactivates the second circuit when the valve is in a first position, and activates the second circuit and deactivates the first circuit when the valve is in a second position. To purportedly improve the steering behavior, the working connections of the second circuit are connected with a pressure source when the changeover valve is in the first position, and the working connections of the first circuit are connected with a pressure source when of the changeover valve is in the second position.
In previously known steering control systems having redundant or backup control circuits, the components of the primary control circuit are provided as a first valve group contained within a first housing, and the components of the redundant control circuit are provided as a second valve group within a second housing. The first and second housings combine to have a significant amount of weight that must be moved by the engine along with the other components and systems of the machine, and claim a significant amount of space within the engine compartment or other compartment within which the steering control system is housed. Moreover, the first and second housings are placed in fluid communication by attaching external hoses there between to transfer steering and/or pilot fluids between the housings so that either the primary or the redundant control circuit can control the operation of the steering components. The points of attachment of the external hoses to the housings create potential leak points in addition to leaks that may develop along the hoses.
In either the primary or the redundant control circuits, fluid flow through the steering control system does not occur when the machine is not being steered by the operator, even when the engine and fluid pumps are running. At startup, pilot valves of the steering control system are typically biased to closed positions where fluid flow through the pilot valves to the steering valve is prevented. While other fluids are circulating in the hydraulic systems of the machine and warming up prior to operation of the machine, the fluids in the steering control system remain stagnant and do not warm up. At cooler temperatures, the fluids in the steering control system may be more resistant to flow, resulting in decreased responsiveness when the operator begins to drive the machine and command the steering control system to move the steering components of the machine. While the primary control circuit operates, the fluids in the redundant control circuit remain stagnant such that the same issues of system responsiveness may be present when steering control is transferred from the primary control circuit to the redundant control circuit.
In view of this, a need exists for an improved electro-hydraulic steering control system providing reduced weight, space claim and leak points. A need further exists for a steering control system having improved fluid circulation when steering of the machine is not being performed, and correspondingly improved responsiveness during cold startup and redundant system cutover.